Brucella Genomes

The B. melitensis strain 16M (Biovar 1, ATCC 23456) genome was the first to be sequenced, closed, and annotated from among the six Brucella nomen species.1-1-4 This was followed by the genome of B. suis strain 1330 (Biovar 1).[5] The total genome size of B. melitensis is 3.30 Mb, similar to that of B. suis (3.31 Mb).

The genome of B. melitensis is distributed over two circular chromosomes of 2.12 and 1.18 Mb, whereas that of B. suis is distributed at 2.11 and 1.20 Mb.[4,5] Their chromosome maps can be accessed at NCBI (http://www.ncbi.nlm.nih.gov/genomes/MICROBES/ Complete.html).

Some general features of the B. melitensis and B. suis genomes are presented in Table 1. The B. melitensis genome encodes approximately 3197 ORFs whereas that of B. suis encodes 3388 ORFs. A GC content of 57% was noted for both species. In B. melitensis, the origins of replication are similar to those of other a-Proteobacteria. Housekeeping genes, including those involved in DNA replication, transcription, translation, core metabolism, and cell wall biosynthesis, are distributed on both chromosomes. Type I, II, and III secretion systems are absent, but genes encoding sec-dependent, sec-independent, and flagella-specific Type III, Type IV, and Type V secretion systems as well as adhesins, invasins, and hemolysins were identified.[4,5]

B. melitensis and B. suis genomes shared extensive similarity and gene synteny. Greater than 90% of their genes showed 98% to 100% identity at the nucleotide level. Those genes with less than 95% identity included mostly the hypothetical genes, the UreE urease component, the outer membrane proteins, membrane transporters, a putative invasin, and Shd-like adhesins. These genetic differences may significantly contribute to the differences in pathogenicity or host preference between the two organisms. A total of 7307 single nucleotide polymorphisms were noted between B. melitensis and B. suis. This interspecies frequency is lower than some of the intraspecies frequency reported in some other sequenced organisms.[5] Based on whole genome nucleotide alignments, 42 B. suis and 32 B. melitensis genes are completely absent from each other's genome. This difference may be due to insertion or deletion events that possibly took place during the course of the species evolution.[6,7] Overall, analysis of the Brucella genomes has not revealed any traditional virulence factors, toxin genes, or pathogenicity islands.[8] A study of the virulence mechanism of Brucellae is currently an active area of

Table 1 Comparative general features of the B. melitensis and B. suis genomes

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